A quadrangular castle is a strongly fortified residence, of masonry or sometimes brick, built around a square or rectangular courtyard (usually known as a ward). The outer walls formed the defensive line known as the enceinte, and towers would almost always be sited on the corners and sometimes in intermediate positions too. Some of the very strongly defended castles of the class have an additional wall, set a small distance away from the main wall, and concentric with it, and can be classified as concentric quadrangular castles; the area between the two walls is the outer ward. Moats, usually wet but sometimes dry, were often formed outside the walls. Accommodation within the castle was provided in the towers or by buildings set against the walls and opening onto the ward.

Sites that may be confused with this class are those where concentric defences have been developed round earlier buildings; a good example of this is the Tower of London, where concentric walls have been developed, at different times, round an inner ward containing a tower keep.

Excluded from this description are fortified homesteads, often built as a quadrangle and surrounded by a moat, which although meeting many of the criteria for a quadrangular castle, are usually smaller and have little or no strategic function; they are invariably less defensible.

Quadrangular castles were the strongly defended residence of the king or a lord, sited for offensive and defensive operations, and often forming an administrative centre; they are found in urban and rural areas. They incorporated the elements of scientific fortification as it developed in the 13th century and the finest examples in Britain are undoubtedly those of Edward I in Wales, built between 1277 and 1295. Examples also appear in England in the late 13th century but most were built in the 14th; the class continued to be built until the late 15th century.